Depends on the woman of course (and the man), but I generally prefer bikinis on women, and trunks on guys. One man in a hundred (at most) looks good in a speedo! And even if he looks good, he still looks like a euro dork.

For myself, I prefer to wear a bikini no matter if I'm not one with a "conventional 'bikini' body." They work the best on my body.

I've seen many one-pieces that are very pretty, but they just don't work for me. There's no support, so my boobs are just flattened into nothingness. And then they usually are not cut for my shape. If it fits my chest and hips, the fabric of the suit never even touches my waist because it's cut so wide, so I look like a big rectangle.

Tankinis are okay, but they tend to just ride up over my hips and bunch around my waist, which is really uncomfortable.

I prefer to wear one pieces; bikinis always seemed too close to being in your undergarments for my liking. I like seeing women in these new monokinis and tankinis though, they look so neat!
Men, boxer style.